1. Field of the Invention
This invention relates to ultrasonic transducers for use in noncontacting distance measurement and profile detection systems for any solid object in air.
2. Description of the Prior Art
As is well known, piezoelectric ceramic transducer elements or magnetostriction transducer elements have been used in ultrasonic air transducer arrays. These elements may be broadly divided into three types with respect to construction.
In one such construction, a piezoelectric or magnetostriction transducer element is integrally combined with a metallic horn at one end, which is in turn combined with a metallic vibrator plate of a relatively large area at the other end of the horn. The use of the metallic vibrator plate of a relative large area serving as an ultrasonic radiating surface enables one to achieve, to an extent, an acoustic impedance-match between the piezoelectric or magnetostriction transducer element and the air.
Another type of construction comprises a bimorph piezoelectric transducer element capable of flexural vibrations and a thin aluminium cone connected to the transducer element through a bar. The transducer is so designed as to match the acoustic impedance between the piezoelectric transducer element and the air with the aid of the cone.
In the above prior art transducers, the flexural vibrations of the metallic vibrator plate or the bimorph piezoelectric transducer or the cone are utilized and thus it is almost impossible to raise the resonance frequency. These types of transducers have been ordinarily used only to generate ultrasonic waves in air below 100 kHz. Such a relatively long wavelength in air is not satisfactory for distance or azimuth resolution or profile or nature resolution.
Moreover, these known transducers make use of the flexural vibrations and have a difficulty in phase control of ultrasonic wave radiated into the air. This leads to the difficulty in controlling the directivity of the ultrasonic beam.
A further transducer makes use of thickness vibrations of a piezoelectric transducer element. The transducer element has an acoustic impedance-matching layer on the ultrasonic wave transmitting front surface thereof. On the back surface of the element is formed a backing layer. In order to match the acoustic impedance between the piezoelectric transducer element and the air, the matching layer is made of a composite material comprising an epoxy resin or silicone resin matrix and microspheres of glass having a diameter of several hundreds microns or below.
As regards the magnitude of acoustic impedance, when a PZT piezoelectric ceramic is applied as the transducer element, the sound velocity, v.sub.1, of the element is about 3500 m/sec., and the density, .rho..sub.1, is about 8000 kg/m.sup.3. The acoustic impedance, Z.sub.1, represented by the product of the sound velocity and the density is about 3.times.10.sup.7 Ns/m.sup.3. On the other hand, the acoustic impedance, Z.sub.2, of air at a normal temperature is about 400 Ns/m.sup.3. With the construction using only one impedance-matching layer, the acoustic impedance-matching layer should have an acoustic impedance, Z.sub.m, ##EQU1## That is, Z.sub.m =0.11.times.10.sup.6 Ns/m.sup.3. In the case, the acoustic impedance-matching layer has preferably substantially a quarter wavelength thickness.
The acoustic impedances of conventionally used silicone and epoxy resins are, respectively, 1.0.times.10.sup.6 Ns/m.sup.3 and 3.0.times.10.sup.6 Ns/m.sup.3. These values are larger by one order of magnitude than the acoustic impedance obtained from the equation (1). Satisfactory matching between the element and the air cannot be achieved, so that the sensitivity of the transducer lowers.
With the acoustic impedance-matching layer in which hollow microspheres of glass are distributed throughout a synthetic resin matrix, the density, .rho..sub.g, of the glass microspheres is about 300 kg/m.sup.3 and the density, .rho..sub.o, of the resin matrix is about 1000 kg/m.sup.3 when using silicone resin. When the weight ratio of charged hollow glass microspheres is taken as r.sub.m, the density, .rho., of the resulting composite material is expressed by the following equation (2) ##EQU2##
The density, .rho., in relation to r.sub.m varies as shown by the solid line curve of FIG. 1. In the figure, indicated by a broken line curve is the relation between the weight ratio and the volume ratio, r.sub.v, of the hollow glass microspheres in the total composite material. The volume ratio, r.sub.v is represented by the following equation (3) ##EQU3##
As will be seen from the figure, when the weight ratio of the microspheres is, for example, 0.30, the volume ratio is 0.59. The composite material comprising such microspheres has a density of 590 kg/m.sup.3. An increased value of r.sub.m results in a smaller density, .rho., of the composite material with an increased volume ratio, r.sub.v, of the microspheres being charged. Uniform mixing and charging of the microspheres is thus difficult.
Hollow microspheres of glass having a density of 300 kg/m.sup.3 are mixed with a silicone resin having a density of 1000 kg/m.sup.3 in different ratios to determine a density and sound velocity thereof. The results are shown in Table 1 below.
______________________________________ Weight Ratio of Hollow Glass Micro- Density of Sound Velocity Acoustic spheres Mixture of Mixture Impedance ______________________________________ 0.15 740 kg/m.sup.3 1300 m/sec. 0.96 .times. 10.sup.6 Ns/m.sup.3 0.30 670 kg/m.sup.3 1500 m/sec. 1.01 .times. 10.sup.6 Ns/m.sup.3 ______________________________________
As will be seen from Table 1, an increased weight ratio of the microspheres is not so effective in lowering the acoustic impedance. More particularly, the acoustic impedance values of the composite materials are larger by one order of magnitude than the acoustic impedance calculated from the equation (1), i.e. 0.11.times.10.sup.6 Ns/m.sup.3. Thus, such composite materials are not suitable when applied as an acoustic impedance-matching layer.
Ultrasonic transducers comprising two impedance matching layers are known for use in medical ultrasound examinations. The guiding principle in the design of such ultrasonic transducers has been reported, for example, by Fukumoto et al ("National Technical Report", Vol. 29, No. 1 (1983), p. 179). In this report, acoustic impedances necessary for the respective impedance-matching layers are determined based on analytical and numerical techniques using the respective two equations. For instance, when a PZT piezoelectric ceramic transducer element is used, the first acoustic impedance-matching layer on the element surface and the second impedance-matching layer on the first layer are determined, according to the respective equations, to have acoustic impedances of 1.8.times.10.sup.6 Ns/m.sup.3 and 6.9.times.10.sup.3 Ns/m.sup.3, or 0.25.times.10.sup.6 Ns/m.sup.3 and 2.times.10.sup.3 Ns/m.sup.3.
However, materials for existing impedance-matching layers have an acoustic impedance of at most 0.9.times.10.sup.6 Ns/m.sup.3. Thus, the above requirement for the ultrasonic air transducer comprising two matching layers cannot be satisfied.